1. Field of the Invention
The present invention relates to a separating membrane, of plastic material, of the type with bosses with improved adhesion both on the upper face, towards the cement adhesive, and on the lower face, towards the base layer; the present invention also relates to the production process for obtaining said membrane.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
The present invention finds particular application in the building industry and in particular in the sector of semi-finished components for building such as, as a non-exhaustive example, the thin membranes which are intended to separate the screed from the flooring; the membrane according to the invention, more generally speaking, is suitable for the works of civil or environmental engineering, also in combination with other materials.
Nowadays, in the building sector various types of membranes are known, which have various purposes: for example, one should remember the vapour membranes, the bituminous waterproofing membranes, the drainage membranes, the geomembranes or even the separating sheets of nonwoven fabric. In particular, in modern building the waterproofing membranes are widely known, which are commonly applied by the layers of ceramic coatings, of marble slabs or anyway of coatings of the fragile type, in order to prevent cracks and detachments; such problems, in fact, sometimes occur in case of differentiated thermal expansions of the layers, or due to the movements of the floor, due to foundation microsubsidences or for any deformation of the supporting plane which is not correctly followed by the coating layer. A membrane, which is intended to prevent such problems, is conventionally called a separating membrane, or uncoupling membrane or even separating plate, being interposed like a diaphragm between the load-bearing structure and said coating in order to prevent its direct adhesion, to disperse stresses and to absorb deformations; to this purpose, particularly effective are the waterproof membranes of the bossed type which separate more the layers, distribute the point loads in a uniform way and also compensate for the vapour stresses of the subfloor.
Therefore, in professional practice it has been observed that today the most used solutions provide waterproof membranes of the bossed type, used on their own or coupled with a permeable fabric layer. In more detail, widespread are the sheets with small-sized bosses obtained on the surface in a continuous way, like cylindrical protrusions, or protrusions shaped as a truncated cone or a truncated pyramid arranged in a regular matrix, which, being intended to increase the distance between the layers, and also being intended to improve stability and adhesion, allow the cement adhesive, which is in contact with said sheet, to penetrate the interstices between the bosses or the cavities internal to the protrusions themselves, according to the positioning direction.
By way of example, the single-layer bossed sheets are widely used in underground constructions, as an element of external protection and waterproofing from the ground, and also in said cases of ceramic paving on floors of reinforced-concrete, of wood or in the screeds on the ground floor. However, for some applications, for example for a more effective drainage or to increase said separating function, the bossed sheet is also coupled with a base layer of permeable fabric, such as a nonwoven fabric of polypropylene of the type called spunbonded or spunlace; in particular, in the separating use on a reinforced-concrete screed, it is well known that such a base layer adheres well to the underlying adhesive interpenetrating it and also helps to compensate for the differentiated movements between said screed and the flooring above the bossed sheet. For example, see the drawings of the known prior art (FIGS. 1a-c) which represent a conventional separating membrane made up of a waterproof sheet provided with hollow bosses, of a cylindrical shape and arranged in a regular pitch matrix, which is also coupled on the lower side with a base layer of the permeable type of nonwoven fabric.
From the production point of view, a membrane of the bossed type is industrially obtained by extruding a sheet of plastic material which is then formed on a roller whose surface is provided with protruding bosses, which is also called bossed forming roller. In more detail as to said extrusion, there is a gravimetric feeding system which, in an automatic way, doses the raw materials directly on the hopper through which they enter the extruder; it, generally, is a heated cylinder inside which there is a worm screw that transports the melted material to the head section. Therefore, the melting of the plastic material occurs inside said extruder, where the temperature is not uniform, it being generally between 185° C. and 290° C. according to the passage zone of the melted material; the flow of said melted material is then distributed over the entire width of said head, and with the desired weight, by means of particular deviation lips which are spaced apart in an adjustable way according to the desired product. Along the screw a growing pressure is generated, from the hopper to the head, excluding a depression zone induced by a different section of said screw which allows to extract any possible volatile substances; moreover, before being cast around the forming roller, said melted material meets some filtering stations intended to block any possible impurities.
The so melted plastic material by means of a depression is cast and adheres temporarily to the bossed surface of the roller in such a way as to assume the same bossed shape and also to replicate the same smooth surface to in the zones included between one boss and another boss. The shape, the height and the pitch of the bosses which characterize the finished product can be easily varied, by replacing from time to time said bossed forming roller with a specific roller structured in the desired way. Then the so formed membrane passes through some thermal conditioning cylinders, towards an accumulator mount, to be finally conveyed to the cutting, rolling and packaging zone. If a base layer is also provided, said nonwoven fabric can be advantageously coupled during the manufacturing process of the sheet, for example by conveying it already stretched on the just made product in such a way as to exploit the residual heat of the sheet and realize several heat melting points, that is to say, in correspondence of the head of the bosses; as an alternative, it is possible to cold glue the fabric by means of adhesives laid in strips or dots.
The most suitable raw materials for this type of working are the resins of the type called olefins, and in particular polyethylene, which can be low-density, medium-density or even high-density, being called by the LDPE, MDPE and HDPE acronyms, respectively; as an alternative, polypropylene, known by the PP acronym, or thermoplastic polyolefins, known by the TPO acronym, are suitable as well. In some cases polystyrene, known by the PS acronym, is also used. Such raw materials can be either virgin or regenerated, the latter being derived from the recovery of products in the post-production or post-consumption phase.
In general, those skilled in the art know that a membrane with an uncoupling function, particularly of the type with a bossed sheet joined on the lower side to said base layer, generates the above-described advantages but also produces a considerable decrease in the values of adhesion between the flooring and the screed. In particular, both the adhesion between the bossed sheet and the overlying cement adhesive, and the adhesion between said bossed sheet and the base layer of nonwoven fabric are weak; on the other hand, the adhesion of said base layer to the underlying cement adhesive is solid and effective.
In more detail, as to the adhesion of the adhesive mortar to said sheet, it has been observed that the known and conventional solutions available on the market do not allow for a suitable adhesion to the bossed sheet of the bedding layer of the flooring and particularly in the light of the new building regulations, such as the UNI EN 1348 European standard of 2000, which specifically requires each cement adhesive on ceramic tiles to resist at least at 0.5 N/mm2 following the test at 28 days, also called initial adhesion. For example, several tests have shown that a smooth polyethylene sheet, bossed in a conventional way, reaches a maximum tear strength value of 0.15 N/mm2, according to said standard. In fact, it is known that a smooth polyethylene sheet does not allow for an effective chemical adhesion, and furthermore the conventional shape of the bosses, for example shaped as regular discs having a diameter of 20 mm and a depth of 3 mm, does not form a sufficient mechanical grip; such a problem generally is the cause of the localised detachments, which sometimes occur in ceramic tile flooring or anyway in small-sized coverings.
Said UNI EN 1348 European standard of 2000 provides tensile tests on specifically prepared reference samples. The adhesive is spread onto a concrete plate by means of a putty knife equipped with 6×6 mm teeth and spaced apart by 12 mm from the centre, keeping an inclination of 60°; afterwards, said tiles are applied keeping on them a weight of 20 N for 30 seconds. After the maturation of the adhesive, the tensile test is carried out with a force of 200-300 N/s, connecting the tile to an axial tensile instrument by means of interconnection systems which do not entail bending efforts.
In case of interposition of said separating membrane, the preparation of the reference samples occurs by applying on a first bedding the membrane sample being careful to exert such a pressure as to allow for a tight coupling between the cement adhesive and the layer of nonwoven fabric, if present. Therefore, one spreads out the bedding again with a toothed putty knife for the application of the tile on the sticking surface—being careful to fill the chambers of the bosses—which is then pressed with a weight of 20 N for 30 seconds. In particular, the bossed membranes of the type provided by the invention have a substantially smooth plastic sheet, which does not simplify the connection neither to said cement adhesive nor to an optional layer of nonwoven fabric coupled with it. The membranes, which are currently available on the market, reach much lower values with respect to the minimum initial adhesion value of 0.5 N/mm2, a value greater than 0.2 N/mm2 being already considered as good.
Furthermore, it has been observed that in the specific sector of flooring there is no reference standard for assessing the adhesion between a plastic sheet and the base layer of fabric; to this purpose, it is possible to operate by measuring the delamination force with a dynamometer. Substantially, one stresses the interface between said fabric and the bossed sheet in such a way as to originate the reciprocal detachment by delamination. In general, in the absence of regulations intended to regulate this matter, in the case of composite materials one can follow a test method prescribed for the delamination tests, which are also called peel tests, such as the EN544 European standard, which measures the so-called induced delamination. Such test provides to sample in different zones of the finished product some 10×20 cm samples, with the longer side facing the machine, and to stabilize the samples for 24 hours in laboratory, at 25° C. with 50% relative humidity. From an operative point of view, one takes the fabric layer on one side and carries out a delamination for a 10×10 cm area in such a way that the sample is half delaminated and half still adhering to the base fabric; afterwards, one inserts on the lower jaw of the dynamometer the delaminated portion of the sheet while on the upper jaw, spaced by 20 cm from the lower jaw, one inserts the delaminated portion of the fabric.
In more detail as to the measurements, one carries out a traction of the so prepared product, which will be automatically delaminated by the dynamometer by means of the stroke of the upper jaw, which moves at a fixed speed of 100 mm/min. The instrument measures the delamination force which will have a trend based on peaks, reaching the peak every time the product is delaminated in correspondence of a row of bosses; said force, which is also defined as delamination force, is equal to the peak value divided by the width of the sample, being thus expressed in N/mm. In particular, it has been observed that the membranes available on the market generally have values of about 0.1 N/mm; such values exclusively refer to the systems coupled by thermal adhesion because it is known that the glued systems generally provide higher initial values but in time, following the differences in temperature to which such products are submitted during processing, the sticking tends to detach, sometimes giving rise to interruptions of the contact between the elements with the consequent lifting of the tiles.
Recently, the companies operating in the building sector have proposed several solutions of separating membranes, which are also intended to improve the adhesion to the cement mortar by means of simplified gripping; in principle, one considers as conventional and advantageous the bossed structures with undercuts as well, like localized fixing elements once the adhesive has hardened, and also one considers as conventional and advantageous the coupling of the sheet with a permeable layer which penetrates the bedding. For example, among the most effective and widespread bossed membranes on the market we would like to recall the high-density polyethylene membrane solution by the German company Schlüter-Systems KG—Iserlohn, www.schlueter.de—marketed under the name of Ditra, which has bosses shaped as a 13 mm side square-plan truncated cone, arranged in a regular matrix with a 20 mm pitch, of the type overturned in such a way as to obtain a dovetail section, or also we would like to recall the membrane solution by the Norwegian company Isola AS—Porsgrunn, www.isola.no—marketed under the name of Platon, which has cylindrical bosses having a 13 mm diameter, arranged in a regular matrix with a 20 mm pitch, coupled with a fabric layer on both faces. However, it is widely known that such bossed membrane solutions have said problems of initial adhesion or delamination; for example, the experiments have shown that such solutions reach initial adhesion values, or pull-out values, included between 0.20 and 0.25 N/mm2 with delamination peak values of about 0.1 N/mm.
For the purpose of determining the prior art related to the proposed solution, a conventional check was made, searching public archives, which has led to find some prior art documents, among which:
D1: US2006201092 (Saathoff et al.)
D2: US2011232217 (Hartl)
D3: EP2234802 (Becker)
D4: EP2246467 (Sghedoni)
D5: U.S. Pat. No. 7,536,835 (Schlueter)
D6: U.S. Pat. No. 6,434,901 (Schlueter)
D1 and D2 describe a membrane formed by a bossed sheet of the type with simple protrusions shaped as a truncated cone facing downwards, that is to say, with the cavities facing upwards, which is coupled on the lower side with a layer of nonwoven fabric and on the upper face with a net; in particular, in D1 said net is high-resistance and is stretched in a planar way in such a way as to facilitate the penetration of the bedding mortar but hinder its tearing, after hardening, while in D2 said net is coupled in adhesion with the upper face of the sheet for the purpose of increasing the adhesive effectiveness of the mortar by effect of the projections formed by said net on the surface.
D3 proposes a separating membrane formed by a bossed sheet which is coupled on both faces with a layer intended to improve the connection to the cement adhesive, being for example of a synthetic material of the nonwoven fabric type.
D4 describes a separating membrane formed by a bossed sheet coupled on the lower side with a layer of nonwoven fabric, wherein said sheet has truncated cone bosses with cavities facing upwards and wherein the upper face of the sheet is entirely covered with fibres applied by gluing like in flocking. Said sheet is manufactured by rolling using a roller with regular protrusions with a truncated cone shape; said fibres, having a length between 500 and 1000 microns, are applied by spraying in a random manner on the whole surface and on them one optionally applies a predetermined quantity of Teflon®, so as to prevent the capillary movement of water.
D5 proposes a separating membrane comprising a bossed sheet having undercuts, wherein the cavities of the bosses are facing downwards, the lower layer being permeable and being intended to be penetrated by the adhesive mortar from the screed in such a way as to remain gripped on the dovetail undercuts present in some of them, whereas on the upper face said sheet is coupled with a second layer of nonwoven fabric which acts as a planar support for the bedding mortar of the coating. Moreover, it is provided that the lower surface of the sheet has improved adhesion, being for example roughened or equipped with a rough coating.
Finally, D6 describes a separating membrane comprising a bossed sheet having undercuts, wherein the cavities of the bosses are facing upwards to be filled by the adhesive mortar of the bedding applied over it and are equipped with protrusions which protrude horizontally from the edge of the mouth towards the centre of each cavity, like a barrier, to hinder tearing once said mortar has hardened.
Therefore, it is reasonable to consider as known the solutions of separating membranes for building which are intended to separate the screed from the coating, both said membrane and said coating being laid on an adhesive mortar bedding, which comprise:                a waterproof sheet of high-density polyethylene of the roller bossed type in such a way as to form protrusions, generally of a cylindrical, truncated cone or truncated pyramid shape with a square base, which form cavities on the upper face into which the bedding adhesive mortar can penetrate;        a permeable layer of nonwoven fabric which is coupled on one side of said sheet for the purpose of joining the bedding in an integral way;        means for increasing the adhesion of the adhesive mortar to the sheet made by means of a net stretched or coupled in adhesion on the whole surface of the face, or a fabric;        means for increasing the adhesion of the adhesive mortar to the sheet, with mechanical gripping, made by means of undercuts protruding from the mouth of the cavity or by means of a high-resistance net stretched over them;        in principle, also known is the solution of improving the tear strength of an adhesive to a polymer material in sheets or plates by increasing roughness or wrinkledness, in such a way as to increase the total area corresponding to the contact surface.        
To conclude, we have observed that the known solutions described have some drawbacks or anyway some limits.
In general, as described above, in the conventional and known solutions it is possible to improve both the adhesion between the bossed sheet and the overlying cement adhesive, and the adhesion between said bossed sheet and the underlying layer of nonwoven fabric.
Secondly, it has been observed that a net applied in a planar way on the upper face of the sheet, as for example in D1, effectively increases tear strength but also hinders the penetration of the adhesive mortar into the cavities, with a high cost of the product and with greater complexity of application.
Thirdly, it has been observed that the solutions with fibres made adherent to the sheet, such as the net of the conventional type as in D2 or the particular flocking as in D4, realize micro-grips of the type uniformly spread over the entire contact surface. However, it has been observed that in such cases production is not easy; in particular, for said flocked fibres one must first apply the adhesive, then spray the fibres and also, sometimes, apply a protective substance. Such a process is thus considerably longer and more expensive with respect to the conventional bossed membranes without said fibres on the upper face, also with greater costs of the materials and greater environmental costs. Furthermore, the known solutions in principle provide that the lower surface of the sheet has improved adhesion, as for example in D5, where it is generically proposed to obtain said improvement by means of a roughened surface or a surface with a rough coating; to this purpose, however, it is not described and it is not known how to industrially realize said roughening on the smooth surface of a bossed membrane, and particularly in order to obtain an effective adhesion to the adhesive, with low costs and for large production volumes.
Fourthly, in the known solutions of bossed sheet also with undercuts in the cavities, as for example in D5 and D6, it has been observed that the manufacturing process for obtaining them is not easy, since the processing cycle is longer and more complex with greater waste and also with greater investments in the equipment. The experiments have shown that, for the purpose of gripping, the protrusions constituting said undercuts are generally placed on the edge of the chamber, at the mouth, like a necking which effectively hinders the extraction of the adhesive once it has hardened but which also hinders its entry, during laying; such a configuration in professional use requires time and attention in order to prevent the incomplete filling of the chambers. Therefore, to this purpose it has been observed that a lower positioned protrusion, with respect to the mouth, is more effective, in such a way as to create a sort of guide for the adhesive to enter the chamber and facilitate the complete filling under the necking; moreover it has been observed that, for the purpose of gripping, a thin section of the restraint below said necking is sufficient, with a lower quantity of material to be introduced, while the addition of said guide at the mouth above said necking effectively contributes to increasing the contact surface and, as a consequence, the gripping.
Furthermore, it has been observed in the solutions of bossed sheet coupled with a layer of nonwoven fabric that the latter adheres to the cement adhesive also having said delamination problems and that such problems are reduced by increasing the contact surface with the fabric and increase by decreasing said contact surface. Thus, the experiments have shown that the most widespread solutions on the market have a contact area in a percentage with respect to the whole face of the membrane and corresponding to the heads of the bosses joined to the fabric which is generally between 33% and 49% of the total face; such values are reported respectively to said solutions with square bosses having a 13 mm side and with circular bosses having a 13 mm diameter, with a 20 mm pitch, respectively. Furthermore, it has been observed that it is not advantageous to increase said contact area too much because this increases said delamination values but causes a worsening of said values of initial adhesion on the opposite face; to this purpose, the experiments have shown that, on the other hand, it would be excellent to have a percentage of area of contact with the fabric, that is to say, corresponding to the heads of the bosses on the lower side, which is about between 50% and 75% of the total area of the face.
Hence the need for the companies of the sector to find solutions which are more effective with respect to the existing solutions; the aim of the present invention is also to solve the described drawbacks.